<!DOCTYPE html>
<template><style>
:host {
    display: block;
    background: blue !important;
}
</style><content></content></template>
<script>
(function() {
    var d = document.currentScript.ownerDocument;
    var proto = Object.create(HTMLElement.prototype);
    proto.createdCallback = function() {
      this.attachShadow({mode: 'open'}).appendChild(this.ownerDocument.importNode(d.querySelector("template").content));
    };

    window.MyElement = document.registerElement('my-blue', { prototype: proto });
})();
</script>

